Hi JJB,
This driver is in the "Pololu" family, and have the same feature of configuration (as we name it) as the one used in Ultimakers late family 3D printers (as far as I'll know). Actually your "hack" is just the same thing as I did with my printer, that is using "PoLolu A4988".
You said it improved your print about 80%, yes that was missing step, -but then there is some sign of it still, right. When things improve like this, we might wonder; is there still some missing steps? No there is not! So what could it be then, the answer is "production tolerances". It is maybe a cheap (Chinese?)